Re: UT v.s. UTC in Guide8b

toadatrix May 26, 2004

Thanks Bill, that was an excellent explanation. But it prompts a
follow up question which may be best explained by a hypothetical
example.

Let's suppose that I am trying to use Guide to predict a total solar
eclipse 800 years in the future. I know that at some point the
ephemeris data for the earth and moon start to break down, but let's
assume that they are pretty accurate for at least the next 800
years. When I set Guide to show UT as the time (which you have
explained is UT1) Guide must be making an assumption about the
relationship between dynamical/ephemeris time, as used in determining
the location of the earth and the moon, and UT. So isn't Guide
really using deltaT in this calculation? Also, when it comes to
predicting the local circumstances of an eclipse Guide must have to
make an assumption about deltaT to know the point on the earth's
globe is which is actually facing the sun at any given time.

I guess what I am really asking is -- doesn't Guide always have to
make assumptions about deltaT to predicit future eclipses? The
further out in time we go isn't there an increased danger that the
points on the earth's surface Guide predicts will be in the eclipse
path may be incorrect simply because a faster or slower deltaT value
will place a different point under the moon's shadow cone at that
time.

Like you, I am obsessed with accuracy in these calculations, even
though I realize at least some of the limitations in the process. My
reaction is that it would be great to be able to distinguish UT0 from
UT1 even though almost nobody else would care.

--- In guide-user@yahoogroups.com, Bill J Gray <pluto@p...> wrote:
> Hi John,
>
> The problem is in Guide's table for Delta-T. It currently
looks like
> this, with data up to 1 January 2004 coming from actual
observations
> and the 2006, 2008, and 2010 data coming from USNO predictions
made
> in 2001:
>
> Year Delta-T
>
> 1990 56.8554
> 1992 58.3093
> 1994 59.9847
> 1996 61.6287
> 1998 62.9659
> 2000 63.8285268
> 2002 64.2998152
> 2004 64.5736400
> 2006 67.6 +/- 2.
> 2008 68.9 +/- 3.
> 2010 70.2 +/- 5.
>
> The problem comes from those predictions, made at a time when
> Delta-T was steadily increasing at about a second a year. USNO has
> discontinued these predictions, so I can't replace those values
with
> something more current. For dates near the present, Guide is
> interpolating between the (perfectly reasonable) 2004 value and the
> (now not-so-reasonable looking) 2006 value. The end result is:
Guide
> thinks Delta-T is growing pretty briskly, so it's getting out of
sync
> with UTC.
>
> Fortunately, there is a way around this problem. We can tell
Guide
> what formula to use for Delta-T, as described at
>
> http://www.projectpluto.com/update7.htm#delta_t_def
>
> What I'd suggest doing is to tell Guide that, for dates from
2004
> on, Delta-T is increasing linearly at its current slow rate. That
> means hitting Alt-J and entering the following:
>
> DELTA_T=2004,2100:64.574,18.62,0
>
> You will still eventually see discrepancies between UT and UTC
that
> are greater than .9 seconds. But attempting to evade such
discrepancies
> does not strike me as especially realistic. It would be more of a
> cosmetic fix than anything resembling "what will actually happen".
>
> More specifically, if someone tells you they used, say,
> Delta-T=64.9 seconds, and you want to replicate their results,
> you could "fix" Guide's Delta-T at a constant 64.9 seconds with
>
> DELTA_T=2004,2100:64.9,0,0
>
> Incidentally, "UT" in Guide means "UT1". (UT0 is time
corrected for
> the observer location on the earth, with the effects of polar
motion
> included. UT1 is a "standardized" sort of UT for a non-wandering
pole.
> UT2 is essentially obsolete. The difference between UT0 and UT1
will
> become crucial if Guide is to be accurate at the level of a
> few centimeters, instead of at the level of a dozen meters. So
far,
> the only Guide user with an interest in that level of accuracy has
been
> its author. I'd like to be able to replicate the laser ranging
> observations made of the mirrors left by the Apollo missions.)
>
> -- Bill